Glaze Pinholing Exploration

yeah, a lot of these firing tips are based on individual experience, mine included.

i have strong doubts about the “oxidation hold of 15-30 minutes” after reduction to reduce pinholes and “even out the glaze”, i’m not claiming that it doesn’t work but my own experience points to other solutions. i’d like to see proof.

The supposed culprit in Sue’s glaze was strontium carbonate. So it may be interesting to recreate that glaze UMF with European materials.

The pinholing glaze I was working with is Woo Yellow and only pinholes when thick.

About culprits in Sue’s glaze… The glaze has very high LOI. We can do nothing about strontium carb, but by simply replacing dolomite with talc and wollastonite we can significantly reduce LOI.
I do not have a small test kiln (after summer I’ll make one), so at the next cone 6 firing, I’ll try both these glaze with soaking at 40 °C below the peak and I’ll document everything.

P.S. I do not like the chemistry of this glaze, but it’s not for this thread

I appropriate your points, but those are not the relevant issues.

  1. Don’t hold this glaze against Sue, It is from John Britt’s mid temperature glaze book. Sue has just been using it to experiment with pinholes as the glazes has a history of pin holing

  2. We’re not interested in altering the glaze’s chemistry, just defining why this exact glaze pinholes. That is why we would recreate the glaze with European materials. To see, if there is something in the American materials that causes the pin holing, or if the glaze recreated with European materials behaves exactly the same.

Some people proclaim that the pin holing here is caused by the degassing of the Strontium Carbonate.

But this chart, shows that SrCO3 degassed long before the glassy stage of the firing, making that an impossibility. So the relevant question is, Is there something in Strontium Carb that influences particle packing and thus, pinholing.

Using David’s definition, is Sue’s glaze pinholing or pitting? Looks like this doesn’t have to do with the clay body as much as bubbles in the thicker parts of the glaze?

My inherent argument, that those are all the same thing.

Thanks Matt for the graph!
I have European strontium carbonate from Colorobbia, CA is: SrO 70%, CO2 30%.
A few days ago, after almost 20 years of struggling, I got CA from Colorobbia of some materials and frites!
I have my opinion about glaze “sponge sindrom” based on observations of glazes through the chimney (and some common ceramic sense) when I had no pyrometer. But, for now:
Altered glaze can prove that the problem is in strontium carbonate.
If it has the same problems as original - the culprit is definitely just strontium carbonate.
I will test both glaze with European materials and report the results.

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I’m very curious about the results of this test. I’m convinced that strontium is contributing to bubbles in some way. I have worked this woo yellow glaze all over the stull map, fixed the flux ratio, added boron and the bubbles are still there. I’ve never removed the strontium though. Even if the pinholes don’t appear on the surface, if I break a tile, the bubbles are within the glaze. I have noticed a reduction in visible pinholes with the reduction of zircopax, but not necessarily an elimination. Seems that zircopax makes it worse. And again, they only appear where the glaze is thick or pooling.

Another glaze we use at my studio is a strontium turquoise matte. It also pinholes sometimes and quite often produces small blisters on edges. Every glaze that I’ve wrestled with pinholes has contained strontium carb. If there was a strontium frit that I could get access to, I’d like to try that. My suppliers don’t carry any.

Here is the turquoise glaze on it’s own, over another glaze, and in reduction. It’s always at the edges of the glaze and not always at the bottom, but sometimes on the rim. It does this on all 4 clay bodies we have at my studio - red stoneware, buff stoneware, white stoneware and porcelain. But it doesn’t do it every time. Sometimes it’s very smooth and lovely.

Here is the UMF of the Turquoise glaze. It contains 2% Lith carb, which I know is a problem, but not sure if it’s contributing to this problem. The woo yellow contains no Lith carb. Colourant is 3% copper carb.

I have a very sketchy idea of what happens on a microscopic level when a glaze melts, so my ideas may be completely off-base, but thought I’d raise the following questions anyway:

Is the volume of a particle of SrCO3 is greater than the volume of the resulting SrO particle(s) after it thermally decomposes? If it is, would this result in microscopic voids in the glaze, or would the particles just pack together more tightly?

Basically, I’d like to question the assumption that off-gassing of glaze materials can only be responsible for bubbles in the glaze if it happens after some of the glaze materials have started to melt. Can off-gassing change the particle-packing?

Pieter, this sounds like a very intelligent theory. I can’t wait to hear what others think about it.

Here’s an example of bubbles beneath the surface of the glaze

I realize this is probably not helpful and have no clue about your problem, however just now when looking for information about carbonates I came across this message from Pete Pinnell in Clayart. I’m guessing you’re already using the fine ceramic grade version though…

If I were you, I would return that stuff to the supplier. There are many
different grades of strontium carbonate available to them, and your
supplier is stocking some rather poor stuff. A good, ceramic grade
strontium carbonate should be a bright, fine, white powder that looks
like a good grade of whiting and passes easily through a 60 mesh screen.
If it doesn’t, send it back.

Since I often use strontium carbonate in glaze formulas, I get many
inquiries from potters when their glazes don’t work. Inevitably, the
problem turns out to be a coarse or impure grade of strontium. Just out
of curiosity’s sake, I have tried ball milling some of the coarse, tan
colored strontium that is sold, and it still didn’t work- which leads me
to believe there must be other impurities involved.

Besides the difficulty in getting the stuff through a screen, there are
a couple of other glaze flaws that seem to stem from this stuff,
including pinholing, and excessive running. For instance, my Weathered
Bronze glaze should not run at cone 9 reduction, and should not develop
pinholes on a clean clay body, two problems I often see when others use

Sometimes I feel like that announcer at Woodstock: “Hey people, there’s
some bad strontium out there!” (Am I showing my age with this arcane

Pete Pinnell

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